The measurement of various respiratory parameters, such as resistance to air flow through bronchial tracts, elastance of pulmonary tissue, and so forth, is essential in the diagnosis of many pathological respiratory conditions, whether these are chronic or temporary.
Medical science uses many different methods for determination of respiratory parameters, some of which are disclosed in the patent literature. For example, U.S. Pat. No. 4,036,222 teaches an apparatus for continuous measurement of the resistance and elastance referred to supra. These are continuously monitored and displayed, and show respiratory air flow and endothoracic pressure variations in the subject.
U.S. Pat. No. 4,031,885 teaches measurement of some different values, i.e., lung pressure and compliance, as well as resistance. The apparatus disclosed therein is used in connection with a respirator system. Compliance and resistance are determined by measuring the pressure of the respirator system at the end of patient inspiration and at the end of plateau phase. The measurements are related to the volume of gas delivered to the patient, and then computing compliance and resistance from the volume, pressure, and time differential values, using known equations relating these values.
U.S. Pat. No. 4,259,967, describes an apparatus useful in measuring respiratory resistance. The apparatus contains a closable valve which permits or blocks passage of exhaled air from the subject. The passageway through which the exhaled air passes also contains means for measuring the rate of flow, or pressure, of exhaled air. There is no way, however, for relating the measured pressure to the volume of air exhaled, and hence no way of determining whether there is a diseased state in the subject's respiratory system, when the disease is characterized by particular relationships between inhaled volume and exhaled pressure.